It took till September 1981 that R107 SL received a heating system with pulsed mono or duo valves from Bosch, right hand steering cars even never received it. W126 was already equipped with this system since it was launched and kept it till its end, W123 received it in August 1980 but only for automatic climate control. Pulsed valve inserts were specified to operate 4 million times but they never made it to that spec. That is why these pulsed heating systems often fail or have a bad control of temperature. Here you learn how to repair this fault.

When I let Sternzeit-107 (startime-107) users in spring 2018 vote which workshop they would like to have the outcome was a heating and A/C workshop for the 107 model. I was already very familiar with all systems up to August 1981 Manual systems with 4 operating levers (till end of 107 on right hand cars) and the first series automatic A/C control system I already knew by heart. With operating levers hot engine water is either blocked or guided through heat exchanger at full flow. Temperature control is done via flaps guiding cold air on both sides above and below around heat exchanger. This control means you adjust it and have to readjust levers if it gets too hot or too cold, it is an open loop control ignoring engine or environmental temperature. Closed loop control means that system measures these temperatures and inside temperature and adjusts automatically. This was done in 1. series automatic A/C control via heater servo in 107, 123 and 116. This servo controls the hot water flow into heat exchanger and has a temperature dial. It is the sample that this pulsed water flow in W126, as of 8/80 in W123 (only automatic A/C) and R/C 107 as of 9/81 (with the exception of right hand steering cars) both in heating automation or A/C automation is based upon. Heater servo is replaced by a mono or duo valve that opens water flow for longer or shorter periods depending on temperature deviation from desired inner temperature. I had to study the whole system before this workshop and contacted one of its developers. He had only constructed the operation panels but he knew the whole system of course very well and helped me understand it. And big surprise, I could see that both valve inserts in my W126 were already broken. But let us not jump to conclusions yet!

When do I have a mono valve and when a duo valve?

Mono and duo valves were launched as of 9/81 in 107 model and then only for left hand steering. So 107 was the last to receive this pulsing valve technology after W126 (all cars) and W123 automatic A/C control. You will notice that operating levers for heating control are then gone, instead you have one or two dials for temperature selection plus dials or buttons to select air guidance. There are 3 versions of this pulsed system: